1. Field of the Invention
The present invention relates to an air-fuel ratio control system which is capable of accurate control of the air-fuel ratio regardless of a change in the characteristics of an oxygen concentration detector in the negative feedback control of the air-fuel ratio by detection of the oxygen concentration of the exhaust gas for purifying the exhaust gas of an automobile internal combustion engine.
2. Description of the Prior Art
The purifying ability of the catalyst is very high when the air-fuel ratio of the mixture gas is at or near the predetermined stoichiometric air-fuel ratio (air excess rate .lambda.=1). In view of this, in a conventional system suggested for purifying the exhaust gas by use of a three-way catalyst in the exhaust system of the internal combustion engine, the oxygen concentration in the exhaust gas representing the air-fuel ratio of the mixture gas is detected by an oxygen concentration detector (hereinafter may be referred to as the oxygen sensor) and the air-fuel ratio of the mixture gas is controlled to about the stoichiometric air-fuel ratio by negative feedback. The output characteristics of this oxygen sensor, however, greatly vary with time or according to production processes. In the case where this oxygen sensor is placed upstream of the three-way catalyst in the exhaust system of an internal combustion engine, the output characteristics thereof in relation to the air-fuel ratio vary between the oxygen sensors S1 and S2, for example, as shown in FIG. 1A. Therefore, even if a reference level Vs is set corresponding to the target stoichiometric air-fuel ratio (.lambda.=1) for negative feedback control, an accurate control of the stoichiometric air-fuel ratio may be attained for the oxygen sensor S1, while an air-fuel ratio lower than the stoichiometric air-fuel ratio is attained for the other oxygen sensor S2, thus resulting in the loss of the effect of the three-way catalyst.
In the case where the oxygen sensor is disposed downstream of the three-way catalyst, on the other hand, it was confirmed that as shown in FIG. 1B, the curves of the detection voltage characteristics with respect to the air-fuel ratio pass the set reference level Vs at or near the stoichiometric air-fuel ratio. In this way, the oxygen sensor may be placed downstream of the three-way catalyst for negative feedback control of the air-fuel ratio, but in this case it is apparent that a system response delay occurs unlike in the case where the oxygen sensor is placed upstream of the three-way catalyst.